I've had my JB4 installed on my N55 535 for a few months now without any issues. The car has only 1800 miles on it and the tune has been good until this weekend. It's also much colder this weekend so not sure if it has anything to do with it. During warm up if I give the car slightly more throttle the car stutters and sounds like the exhaust is backfiring when it hits over 4k rpm and then goes back to normal. I made the car do it several times and so far only seems to do it before the car is fully warmed up.